Xepoch Xepoch - 8 months ago 67
Linux Question

How does cron internally schedule jobs?

How do "modern"

daemons internally schedule their jobs? Some
s used to schedule a run every so often via
. So after a crontab is written out, does

  1. Parse the crontab for all future events and the sleep for the intervals?

  2. Poll an aggregated crontab database every minute to determine if the current time matches the schedule pattern?

  3. Other?


Answer Source

A few crickets heard in this question. Good 'ol RTFC with some discrete event simulation papers and Wikipedia:


The algorithm used by this cron is as follows:

  1. On start-up, look for a file named .crontab in the home directories of all account holders.
  2. For each crontab file found, determine the next time in the future that each command is to be run.
  3. Place those commands on the Franta-Maly event list with their corresponding time and their "five field" time specifier.
  4. Enter main loop:
    1. Examine the task entry at the head of the queue, compute how far in the future it is to be run.
    2. Sleep for that period of time.
    3. On awakening and after verifying the correct time, execute the task at the head of the queue (in background) with the privileges of the user who created it.
    4. Determine the next time in the future to run this command and place it back on the event list at that time